Recent commercial applications in the art of latent image printing and development have related primarily to teaching aids and self-instructional material in which, for example, a printed question or problem is presented along with a multiple choice of answers, only one of which is correct. Associated with one or more of the answers is an invisibly printed indication of its correctness, adapted to show a color change when contacted with a marking instrument containing a chemical component adapted to react with the invisible ink. These and other applications of latent image printing and development are disclosed in U.S. Pat. Nos. 3,363,336; 3,363,337; 3,363,338; 3,451,143; and 3,516,177.
A wide variety of invisible but color producing systems have been proposed for such uses. The oldest known invisible inks are those responding to heat. Such inks are invisible when applied to a paper substrate but upon application of heat become visible. Examples described in the literature include solutions of ferric sulfate plus phosphoric acid, cobalt chloride, alloxanthine plus oxalic acid, and potassium nitrate. The use of onion and lemon juices as heat sensitive, invisible inks is also disclosed in the prior art.
Another type of invisible ink, the light sensitive ink, has usually been based on dilute solutions of silver salts such as silver nitrate, although other metal salts such as gold trichloride have been used. The use of oxalomolybdic acid has also been reported. Light sensitive inks appear to have found little commercial utilization as invisible inks and no method of adapting such systems to paper-based applications is apparent.
Another class of invisible inks includes those developed by contact with water and consisting of a waterproof, transparent and colorless substance which is applied to a colored or tinted paper substrate. When the substrate is immersed or dampened with water, the area surrounding the inks changes color and the writing becomes visible. On drying, the system reverts back to its original invisible state. Ink systems which may be used include wax solutions, various gums, metallic soaps and other similar, water insoluble substances. While such systems have found wide usage in advertising, a major limitation of such systems is the dependence on the speed of solvent evaporation for reversion to the invisible state.
A relatively new type of invisible ink is one which gives off a bright fluorescence under the influence of ultraviolet light. Many materials are reported which show such fluorescence but most are unsuitable for use in invisible ink systems due to color or solubility problems. Compounds which have been used in fluorescent invisible inks systems include quinine sulfate, esculin, anthracene, and alpha-phenyl-m-aminobenzothiazole hydrochloride.
The class of invisible inks which appears to be best suited to practical commercial application comprises chemically activated inks based on the reaction of two relatively colorless materials to form a colored compound. A wide variety of such chemical systems is disclosed in the prior art and reference may be made to, inter alia, U.S. Pat. Nos. 3,363,336 and 3,438,927 and German Published Patent Application Ser. No. 1,956,143. To be economically suitable for commercial applications, chemically activated invisible ink systems must be nontoxic and safe for use by children, printable using existing printing equipment, provide a vivid color reaction, and should be reusable several times with the paper substrate quickly reverting back to its original state after each use. We have found that invisible inks based upon phenolphthalein, printed on types of paper as defined herein and developed with certain dilute alkaline compositions fulfill the requirements given above.
We are aware that phenolphthalein-alkaline developer systems have long been disclosed in the prior art in, for example, U.S. Pat. Nos. 456,047 and 866,293 and in German Published Patent Application Ser. No. 1,956,143. To date, however, the art has apparently not developed such a system capable of repeatedly being rendered visible with the developed color automatically fading each time within a controllably predetermined period of time.